Ferrihydrite, Lepidocrocite, and Goethite in Coatings from East Texas Vertic Soils

Abstract Iron oxide coatings from ped surfaces and pores of three rice paddy soils and one non‐paddy soil near Beaumont, TX, provided samples for studying Fe oxide mineralogy and the relationship of extractable Fe to extractable Al, Si, and P. Analyses of Fe oxide coatings, concentrated by a combination of scraping, sonication, centrifugation, and magnetic separation, showed that these Fe oxide coatings cement clay minerals and quartz particles together. Iron oxide coatings in all soils contained lepidocrocite and smaller amounts of goethite. An improved differential x‐ray diffraction (DXRD) method, in situ DXRD on a nonreflecting quartz plate, allowed the identification of ferrihydrite, which is otherwise difficult to identify because of its poor crystallinity and low concentration. Most of the ferrihydrite was dissolved by 10 min in pH 3 ammonium oxalate in the dark (AOD). Sequential 10‐, 50‐, and 180‐min AOD treatments extracted lepidocrocite of progressively increasing particle size. The 180‐min AOD treatment extracted minimal amounts of goethite. The DXRD and chemical data indicate that the paddy soils have more ferrihydrite in relation to total Fe oxides than the non‐paddy soil. The P/Fe ratio decreased with each AOD treatment step for all soils. This study demonstrates that physical separation and in situ AOD treatment can be used with the DXRD method to identify ferrihydrite, lepidocrocite, and goethite in soil clays containing extractable Fe concentrations as low as 10 to 44 g/kg. We concluded that a short incremental AOD treatment (≈10–30 min) is a better approach to DXRD of ferrihydrite in <2‐µm fractions of seasonally reduced soils than the much longer treatment recommended in the past for soils.